The invention relates to a colour display tube comprising in an evacuated envelope composed of a neck, a cone and a display window, an electron gun system of the O.L.F.-type (O.L.F.=Overlapping Lens Field) by which three electron beams are generated. The beams are situated with their axes in one plane and are focused on a display screen provided on the inside of the display window by means of focusing lenses. The focusing lenses are generated between apertured, plate-shaped parts of electrodes extending perpendicularly to the beam axes by applying suitable potentials. The apertures communicate with each other so that one elongate aperture is formed in each electrode, and correction means for correcting the focusing lenses are provided in at least one of the electrodes.
Such a colour display tube having an electron gun system of the O.L.F.-type is known from I.E.E.E. Transactions on Consumer Electronics, Vol. C.E.- 26, August 1980, pages 452-458. The focusing properties of focusing lenses deteriorate as their diameter decreases. In an electron gun system having three juxtaposed beams located in one plane the focusing lenses are chosen to be large so as to produce good focusing; the distance between the electron beams must also be chosen to be large. In that case, a glass envelope having a neck of a comparatively large diameter must be used to be able to accommodate the electron gun system. It is more difficult to cause three electron beams which are situated comparatively far from each other to converge on the display screen than three electron beams which are situated close together. The O.L.F. electron gun system described in the I.E.E.E. publication makes it possible to nevertheless use three juxtaposed electron guns with focusing lenses of a large diameter in a colour display tube having a neck of a comparatively small diameter. For that purpose the O.L.F. electron gun system uses overlapping focusing lens fields as a result of which the diameter of the focusing lenses is increased without making the distance between the electron beams larger. The overlapping focusing lens fields are generated with two electrodes in which peanut-shaped apertures are provided in the plate-shaped part which extends perpendicularly to the beam axes. By a peanut shaped aperture is meant an elongate aperture formed by three overlapping circular apertures. Viewed in the direction of propagation of the electron beam, the peanut-shaped apertures are divided, at the area of the constrictions of the peanut-shaped apertures, into three juxtaposed substantially circular apertures by means of separating electrodes. The separating electrodes are metal plates extending perpendicularly away from the plate-shaped part with the peanut-shaped aperture. By providing the separating electrodes with which a correction field is generated, the three focusing lenses formed in this manner are substantially circular so that good focusing properties are obtained. A disadvantage of the above-mentioned construction is that the electron gun systems are difficult to manufacture because the location, shape and orientation of the plate-shaped separating electrodes are very critical.